Orangutans are known to imitate humans. Bonnie, for instance, sometimes sweeps up after herself, just as her caretakers do, even though the zookeepers don't encourage this behavior.

Lead author Serge Wich of the Great Ape Trust of Iowa, said orangutans in Indonesia have been seen pretending to wash clothes.

"We know they are capable of imitating these motor skills, but we never had any good indication of sounds for vocalization," said Wich, who presented his research on December 18 during a symposium at the University of Zurich, Switzerland.

Bonnie the Orangutan Whistling

"Now at least we have an indication that they can imitate sounds" without being trained, he said.